Networks are known which support variable bit-rate transmission by partitioning user data into a sequence of so-called "cells" and inputting them asynchronously to the network. One such network is the Broadband Integrated Services Digital Network (B-ISDN) and is referred to as an Asynchronous Transfer Mode (ATM) network.
ATM networks allow video information to be transmitted with variable bit-rate (VBR). This, in turn, allows statistical multiplexing of data from a relatively large number of users for transmission over a single data channel.
In the ISO Moving Picture Experts Group (MPEG) audio-video coding standard (ISO 11172 Committee Draft of ISO-IEC/JTC1/SC29/WG11), a system multiplexing capability is provided for combining bit streams from several audio, video and/or auxiliary data sources. At a receiver, a system demultiplexer supplies audio packets, including one or more cells, to audio decoders, video packets, including one or more cells, to video decoders and/or auxiliary packets, including one or more cells, to auxiliary decoders. So-called Presentation/Decode Time Stamps (PTS/DTS) in the data packets enable synchronization of the audio and video to be displayed at the receiver. The MPEG standard also provides for a Systems Time Clock (STC) of a predetermined nominal frequency that, in principle, could be locked to a constant rate channel clock. Audio and video clocks could, in principle, also be locked to the STC.
However, in many variable bit-rate or packetized channels there is no continuous channel clock for the receiver to lock to. Moreover, there may be "jitter delay" in the transmission of the data packets. Then, a receive STC must be derived from timing information that is transmitted from a remote receiver. In such arrangements, it is important substantially to eliminate the possibility of data overflow and/or underflow of data buffers in the receiver.
Another complication arises in arrangements in which the receiver video and audio clocks must be more stable than the STC.